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TEA0678T View Datasheet(PDF) - Philips Electronics

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TEA0678T Datasheet PDF : 28 Pages
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Philips Semiconductors
Dual Dolby* B-type noise reduction circuit, automatic
music search, with differential outputs and mute
Preliminary specification
TEA0678
Notes to the characteristics
1. For the signal to be doubled (+6 dB) at differential output as a function of OUTC, the signal-to-ripple ratio is improved
at differential output for approximately 3 dB.
2. By using the small load, the output voltage may be divided by 0.8 dB.
3. The high speed of the tape (FF, REW) at the tape head during AMS mode causes a transformation of level and
frequency of the originally recorded signal. It means a boost of signal level of approximately 10 dB and more for
recorded frequencies from 500 Hz up to 4 kHz. So the threshold level of 22 dB corresponds to signal levels in
PB mode of approximately 32 dB. The AMS inputs for each channel are pin SCA and pin SCB. As the frequency
spectrum is transformed by a factor of approximately 10 to 30 due to the higher tape speed in FF, REW, the
high-pass filter (4.7 nF/24 k) removes the effect of offset voltages but does not affect the music search function.
In the application circuit (Fig.1) the frequency response of the system between tape heads input, e.g. pins INA2/INB2,
to the AMS input pins SCA and SCB is constant over the whole frequency range (see Fig.3). The frequency
dependence of threshold level is shown in Fig.3.
4. In AMS OFF mode, pin AMSEQ is HIGH level, the equalization time constant will be switched by pulling
approximately 200 µA out of pin AMSEQ. This means for the device connected to pin AMSEQ, a restriction of input
current at HIGH level less than 200 µA during AMS off; otherwise the switching of the time constants is disabled but
fixed at 120 µs. If the following devices, input consumes more than 200 µA, this input has to be disconnected in AMS
off mode. (To ensure switching the currents for the different switched modes are specified with a tolerance of ±50 µA
in Chapter “Characteristics”.) For an application with a fixed EQ time constant of 120 µs the equalizing network may
be applied completely external. Change 8.2 kresistor to 14 kthe internal resistor REQ = 5.8 kis short-circuited
by fixing the EQ switch input at the 70 µs position (IEQ70).
Table 1 Blank delay time set by resistor Rt at pin TD
RESISTOR VALUE Rt
(k)
68
150
180
220
270
330
470
560
680
820
1 000
DELAY TIME td
TYP. (ms)
23
42
48
56
65
76
98
112
126
142
160
TOLERANCE
(%)
20
15
15
15
10
10
10
10
10
10
10
1996 Jun 06
9
 

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